IANAL but I think integrating AGPL code would affect the license of the rest of the codebase too: Substack would be forced to release the rest of their codebase as AGPL too. This is why AGPL is considered a "viral" license.
IMHO, the legal definition of "utility" is not necessarily useful. The law is slow, while the real world quickly becomes dependent on pieces of tech. I believe that one day the law will finally catch up and these providers will be likened to utilities.